1. Field of the Invention
The present invention relates to a magnetic brush type developing apparatus for use in developing electrostatic latent images with a toner into visible images.
2. Description of the Prior Art
Generally, a developing apparatus of such magnetic brush type has a magnetic roller disposed in a developing sleeve, the magnetic roller having S-poles and N-poles circumferentially arranged thereon, so that a developing powder is transported on and along the developing sleeve to a developing zone opposite to the surface of an electrostatic latent image carrier in conjunction with the rotation of the developing sleeve and/or the magnetic roller.
One typical recent development in the field of electro-photocopying machines, reader printers, and the like is a developed image producing apparatus of such arrangement that a plurality of developing units are disposed around the surface of an electrostatic image carrier, one of the developing units being loaded with a developing powder including a black toner, the other developing unit being loaded with a developing powder including single-color toners, such as red, yellow, blue, or the like. In another arrangement, one of the developing unit is used for normal development (in which a positive original is for positive development) and the other is used for reversal development (in which a negative original is for reversal development), so that the developing units are selectively switched over from one to the other for development.
However, such apparatus has a limitation that while one of the developing units, so selected for development, is in developing operation, the other developing unit must be so adjusted that the developing powder on the outer periphery of its developing sleeve is kept in non-contact with the surface of the electrostatic latent image carrier in order to prevent the possibility of unnecessary toner deposition and also such possibility that a toner image formed by one developing unit located on the upstream side of the path of movement of the electrostatic latent image carrier surface may be disturbed by the developing powder on the other developing unit which is located on the downstream side of said path of movement.
Therefore, it has been proposed that one of such units which is not selected for development is displaced so that the outer periphery of its developing sleeve is kept away from the developing zone. However, this poses a problem that the arrangement necessarily involves drive means of a larger size for displacing either one of the developing units in its entirety and a larger size supporting mechanism; further, there must be good control of development gaps.
Another proposal is found in U.S. Pat. No. 3,572,288, issued Mar. 23, 1971 to Turner, such that a magnet is rotatably disposed in a separator plate so that when a developing unit is not in developing operation, the magnet is rotated away from a position at which it is opposed to an electrostatic latent image carrier, in order to displace the developing powder from a developing zone. However, such arrangement has a difficulty that since the developing powder is released from a magnetic force of restraint in the developing zone, some part of the developing powder mass falls off the developing unit unless said unit is positioned right under the surface of the electrostatic latent image carrier, which fact necessitates the provision of sufficient preventive measures against fall-off if the developing unit is disposed sidewise.
Similar measures are proposed in Japanese Published Unexamined Pat. Appl. No. 2057/1984, wherein when a developing unit is not in developing operation, a between-poles portion of a magnetic roller is positioned opposite to the surface of an electrostatic latent image carrier. Presumably, this idea may be based on the fact that a magnetic brush of a developing powder tends to rise at magnetic poles, whereas it tends to lie down at a location between poles. Indeed, such arrangement would be effective if employed with a developing powder using a ferrite carrier, in which case a magnetic brush is high enough when the developing powder is at poles, so that there is a substantial difference in brush height between a polar location and a between-poles location. However, where a developing powder using a small-diameter magnetic carrier is employed, the height of such brush is as low as a fraction of that in the former case, there being no much difference in height between a polar location and a between-poles location; therefore, no satisfactory effect can be expected.